Bob Cernik likes using x-rays to probe the nature of materials. The Manchester University professor has been working at the Diamond Light Source synchrotron in Oxfordshire to develop a prototype 3D colour x-ray system to detect hidden explosives, drugs, or even cancer. But wait a moment ... x-rays, in colour and three dimensions?

When Wilhelm Roentgen x-rayed his wife's hand in 1895, he produced a "shadowgraph" showing that x-rays passed more easily through skin than bone. On modern x-ray sensitive film, the shaded areas from bones appear lighter than their surroundings. And, of course, there are no colours we can see.

So how does a hospital x-ray CAT scanner produce colourful 3D body images? It's a process called "false colour", where shades of grey are converted to a corresponding colour in the normal spectrum.

Dense spaces

Cernik, a professor of synchrotron radiation and materials science, cautions against being fooled: "This gives you a high spatial resolution density contrast image that is often false coloured to aid diagnosis. But it is a false colouration that indicates how dense the object is."

There are other x-ray techniques, including diffraction, that allow scientists to identify materials. With this in mind, Cernik talks of "colour" x-rays which, like visible light, contain a range of electromagnetic wavelengths. "Current imaging systems such as spiral CAT scanners do not use all the information contained in the x-ray beam. This extra information can be used to fingerprint the material present at each point in a 3D image."

To do this with x-rays, Cernik's system uses "tomographic energy dispersive diffraction imaging" - or TEDDI. He works with "voxels" (volumetric pixels) which represent points in three-dimensional space. TEDDI measures voxels throughout a sample so that each contains an x-ray diffraction pattern - the key to identifying a material's atomic structure and chemistry.

"You'll get something like a 3D image of the object and you also get some details of the atomic arrangement of every single point," he says of TEDDI's data output. "The system can be programmed to look for something specific, like semtex or cocaine."

Cernik is using the Diamond Light Source's powerful x-rays to develop a solution. He's also relying on advanced detector engineering pioneered at Daresbury Laboratory, Rutherford Appleton Laboratory and Cambridge University, and is helped by funding from the Engineering and Physical Sciences Research Council. "It's a fairly simple idea, but turned out in practice to be a huge technological challenge," Cernik says.

TEDDI requires an x-ray source with a pencil-thin beam, a collimator (to put the radiation into parallel beams), a detector, and much data analysis. The synchrotron provides high energies to penetrate dense metal objects, although Cernik will eventually use compact x-ray sources like the ones used in hospitals.

Making the collimator was precision work: it only allows x-rays travelling parallel to a specific direction to pass through after being scattered by a sample. Helped by Cambridge University, Cernik has laser-drilled 50 micron diameter holes (around half the width of a human hair) 30cm deep through a series of thin (100 micron) tungsten plates. He then matched 256 holes with a 16 by 16 element semiconducting x-ray detector.

In early TEDDI experiments, Cernik imaged thin samples of nylon, aluminium, and deer antler bone. After hours of scanning, he managed to construct images and collect an x-ray diffraction pattern at each voxel. That pattern has between 2,000 and 4,000 data points, potentially gigabytes of data per scanned image.

For larger samples scanned at high resolutions, millions of voxels means a data processing problem but will also provide Cernik with useful answers. "The multiple wavelengths actually contain the information about the crystallography of the object. Our system will identify one chemical component specifically even if they have the same density." Cernik is now using a high energy 80-by-80 element x-ray detector made from cadmium zinc telluride. By tiling together detectors and collimators, he'll make TEDDI run 100 times faster with simpler scanning, lower x-ray doses, and 3D colour imagery.

"We should be able to complete a large scan of a suitcase in about a minute," Cernik says. "We think that one of the biggest applications will be in the security industry." With the right programming, TEDDI might also scan a biopsy sample in seconds to detect cancerous tissue.

"The TEDDI method is highly applicable to biomaterials, with the possibility of specific tissue identification in humans. It could also be used in aerospace engineering, to establish whether the alloys in a weld have too much strain."

X marks the future

Paul Evans, professor of applied imaging science at Nottingham Trent University, thinks that TEDDI is excellent work which produces very accurate results. He's being funded by the US and UK governments to develop the world's first "scatter-enhanced" 3D x-ray security scanner.

His method also uses x-ray diffraction but concentrates on the high-speed identification of substances in cluttered scenes - like the insides of suitcases. X-rays pass through and are scattered by the contents but, compared with the primary beam, the scattered signals are extremely weak.

"If you want to create a security system you've got to solve the problem with weak signals," Evans says. "I'm looking at techniques to produce 3D x-ray images with materials identity information in them."

The system uses sensors to pick up the scattered signals to identify materials and combines them with multiple-view mass discrimination data. Rotating 3D colour images are then presented.

It's a big improvement on airport scanners which "mass discriminate" metals from plastic (and colour the operator's screen for emphasis) but don't tell you the metal. "Using scattered radiation generally enables you to identify materials rather than the crude discrimination which currently exists at airports," Evans says.

Although shadowgraphs and CAT scanners are proven techniques, using those invisible colours in x-rays for materials identification adds a new dimension. Evans now reckons that fast 3D x-ray systems offer another way forward. "People keep thinking we've got to the end of the line with x-rays, but there is a lot to come. High-speed materials identification using x-rays has a rich future."